Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats


YAKUTAT BAY GLACIERS                                     35
Fiord, and Nunatak Fiord; secondary tributaries to these lateral valleys hang above them; and hanging valleys, many with cascading glaciers, lie above the level of the surfaces of all the larger existing glaciers. Many of these glaciers head in cirques, except in the case of the through glaciers. A second evidence of former expansion is the presence of outwash gravels along the shores of the inlet in places where glaciers are no longer present or depositing gravels, and even as far out as the mouth of Yakutat Bay. The third proof is the distribution of transported rock fragments and the development of morainic terraces at elevations high above the level of the inlet, and, where glaciers are present, high above their present surfaces. Such deposits occur all along the shores of the inlet, to the west of Yakutat Bay above the eastern margin of Malaspina Glacier, and in the valleys of the larger glaciers which come down to Yakutat Bay. The moraine terraces descend in the direction of the ocean, and are evidently to be correlated with the deposits \vhich constitute the fourth evidence of former glacier expansion,—the hummocky moraine which forms the southeastern margin of Yakutat Bay, as far out as Yakutat, the similar moraine about the head of Russell Fiord, and the crescentic deposit which extends as a submarine ridge across the outer part of Yakutat Bay.
From these four lines of evidence it has been concluded that, at the period of greatest expansion, all the glaciers were much larger than now,—Malaspina Glacier then rose much higher on the slopes of the mountains west of Yakutat Bay, its tributaries were greater, it received tributaries, notably Lucia and Atrevida Glaciers, that are now disconnected, and it coalesced with a great glacier that filled Disenchantment Bay and Yakutat Bay out as far as Yakutat and the submerged moraine that stretches in crescentic form from Ocean Cape westward to Point Manby. To this expanded glacier that filled Yakutat Bay, the name Yakutat Bay Glacier has been given; and the similar expanded glacier in Russell Fiord has been called the Russell Fiord Glacier. The latter glacier completely filled Russell Fiord and terminated in a piedmont bulb on the inner edge of the foreland, where it has left a crescentic moraine from which outwash gravels slope seaward.
Since the period of maximum glacier expansion, and far more recently than it, there has been a minor advance, by which the united Hubbard and Turner Glaciers, joined by others, pushed southward into Disenchantment Bay and southeastward into Russell Fiord, while Nunatak Glacier, coalesced with Hidden Glacier and others, pushed northwestward into the northwest arm of Russell Fiord, and southward into the south arm about two thirds of the way to the head of the bay. During this advance a lake was formed in the upper end of Russell Fiord whose shoreline can be easily seen and traced. This advance of the glaciers was of such brief duration, and of such moderate intensity, that the ice erosion did not succeed in removing the gravels previously deposited. It, therefore, contrasts strikingly with the earlier, prolonged advance by which the bed rock was scoured out to a depth of many hundred feet by the powerful erosive action of the expanded glaciers. Between these two ice advances there was a long interval during which the glaciers receded even farther than at present, and forest growth extended throughout the fiord, and even up the valleys now occupied by the glaciers. The termination of the last advance was but a short time ago, at the most but a century or two, and the recession from this stage of advance was apparently still in progress as late as 1905. The proof of the recency of the last advance, and of the ice recession from that stand, is the condition of the vegetation growing in the area occupied by the ice and the